Fracturing and propping an oil well is a well known process in which fluid, generally water or oil, is pumped into an oil well at high flow rates (typically 200 to 5000 gallons per minute) and high pressures to hydraulically fracture the underlying oil bearing formation. The fluid is combined with any number of chemicals to produce certain fluid properties. Generally the fluid is mixed with certain polymers to increase its viscosity and allow it to transport a proppant into the fracture created. The fluid is further designed to lose viscosity once it is in the fracture allowing it to leave the porous proppant in the fracture to provide a path for the oil to flow back to the well bore.
To achieve the best performance from the fluid, the various components of the fluid must be mixed together in the proper proportions. One way to verify the composition of the fluid is to measure its density. Because the various components have different densities, the density of the fluid will vary according to the composition of the fluid. It is desirable to measure the density of the fluid before the fluid leaves the blender system, so that the composition of the fluid can be corrected before the fluid is pumped into the well.
Several different types of densimeters are currently used to measure the density of the fluid in the blender system. Some densimeters known in the art use a nuclear gauge which sends a stream of gamma rays through the flowing fluid and determines the density based on the amount of radiation scattered by the fluid. These nuclear densimeters have certain drawbacks, including safety concerns that are always present when working with radioactive materials, and a time lag associated with each density measurement. Also, nuclear densimeters measure the density of only a localized area of the fluid. This is a disadvantage when using a fluid such as a sand slurry, that may not have uniform consistency. When using a nuclear densimeter with such a fluid, the densimeter may measure a localized change in density due to a small pocket of denser material that is not thoroughly mixed into the surrounding fluid. In that case, it is impossible to know whether the changed density is due to a localized concentration of material or a systemic problem with the fluid composition.
Another method of measuring fluid density involves diverting a portion of the fluid flow into a separate U-tube. The U-tube is weighed, along with its contents. The density of the fluid is then calculated, based on the fluid volume contained in the U-tube. While eliminating the safety concerns associated with nuclear densimeters, U-tube densimeters can still provide misleading results when used with inconsistently mixed fluids. Furthermore, the additional tubing needed to construct the U-tube presents a disadvantage, especially in an oilfield environment.
It is the object of the present invention to provide an improved density measuring apparatus which is capable of accurately measuring the density of a fluid flowing in a pipeline and which is not nuclear based. More specifically, it is an object of the present invention to provide a densimeter that can measure the density of a sufficiently large sample of the fluid such that the effect of any local inconsistencies in the fluid composition on the density measurement is minimized. Additionally, it is an object of the present invention to provide a densimeter having a simple, durable structure. It is also an object of the invention to provide a densimeter which produces a density measurement with no appreciable time delay, so that any irregularities in the fluid composition can be identified and corrected immediately.
Further, it is the object of the present invention to provide a method of using the density measuring apparatus to measure the density of a fluid flowing in a pipeline. More specifically, it is an object of the present invention to provide a method of measuring the density of a fluid of unknown density flowing in a closed pipeline system, such as a cementing unit or a blender system for preparing fluid mixtures for fracturing and propping oil bearing geological formations.